We present and analyse the photometric properties of a nearly complete sample of blue compact dwarf (BCD) and irregular galaxies in the Virgo cluster from multi-band SDSS images. Our study intends to shed light on the ongoing debate of whether a stru
ctural evolution from present-day star-forming dwarf galaxies in a cluster environment into ordinary early-type dwarf galaxies is possible based on the structural properties. For this purpose, we decompose the surface brightness profiles of the BCDs into the luminosity contribution of the starburst component and that of their underlying low surface brightness (LSB) host. The latter dominates the stellar mass of the BCD. We find that the LSB-components of the Virgo BCDs are structurally compatible with the more compact half of the Virgo early-type dwarfs, except for a few extreme BCDs. Thus, after termination of starburst activity, the BCDs will presumably fade into galaxies that are structurally similar to ordinary early-type dwarfs. In contrast, the irregulars are more diffuse than the BCDs and are structurally similar to the more diffuse half of the Virgo early-type dwarfs. Therefore, the present-day Virgo irregulars are not simply non-starbursting BCDs. If starbursts in cluster BCDs are transient phenomena with a duration of ~100 Myr or less, during which the galaxies could not travel more than ~100 kpc, then a substantial number of non-starbursting counterparts of these systems must populate the same spatial volume, namely the Virgo cluster outskirts. The majority of them would have to be early-type dwarfs, based on the abundance of different galaxy types with similar colours and structural parameters to the LSB-components of the BCDs. However, most Virgo BCDs have redder LSB-host colours and a less prominent starburst than typical field BCDs, preventing a robust conclusion on possible oscillations between BCDs and early-type dwarfs.
How did the dwarf galaxy population of present-day galaxy clusters form and grow over time? We address this question by analysing the history of dark matter subhaloes in the Millennium-II cosmological simulation. A semi-analytic model serves as the l
ink to observations. We argue that a reasonable analogue to early morphological types or red-sequence dwarf galaxies are those subhaloes that experienced strong mass loss, or alternatively those that have spent a long time in massive haloes. This approach reproduces well the observed morphology-distance relation of dwarf galaxies in the Virgo and Coma clusters, and thus provides insight into their history. Over their lifetime, present-day late types have experienced an amount of environmental influence similar to what the progenitors of dwarf ellipticals had already experienced at redshifts above two. Therefore, dwarf ellipticals are more likely to be a result of early and continuous environmental influence in group- and cluster-size haloes, rather than a recent transformation product. The observed morphological sequences of late-type and early-type galaxies have developed in parallel, not consecutively. Consequently, the characteristics of todays late-type galaxies are not necessarily representative for the progenitors of todays dwarf ellipticals. Studies aiming to reproduce the present-day dwarf population thus need to start at early epochs, model the influence of various environments, and also take into account the evolution of the environments themselves.
We test how well available stellar population models can reproduce observed u,g,r,i,z-band photometry of the local galaxy population (0.02<=z<=0.03) as probed by the SDSS. Our study is conducted from the perspective of a user of the models, who has o
bservational data in hand and seeks to convert them into physical quantities. Stellar population models for galaxies are created by synthesizing star formations histories and chemical enrichments using single stellar populations from several groups (Starburst99, GALAXEV, Maraston2005, GALEV). The role of dust is addressed through a simplistic, but observationally motivated, dust model that couples the amplitude of the extinction to the star formation history, metallicity and the viewing angle. Moreover, the influence of emission lines is considered (for the subset of models for which this component is included). The performance of the models is investigated by: 1) comparing their prediction with the observed galaxy population in the SDSS using the (u-g)-(r-i) and (g-r)-(i-z) color planes, 2) comparing predicted stellar mass and luminosity weighted ages and metallicities, specific star formation rates, mass to light ratios and total extinctions with literature values from studies based on spectroscopy. Strong differences between the various models are seen, with several models occupying regions in the color-color diagrams where no galaxies are observed. We would therefore like to emphasize the importance of the choice of model. Using our preferred model we find that the star formation history, metallicity and also dust content can be constrained over a large part of the parameter space through the use of u,g,r,i,z-band photometry. However, strong local degeneracies are present due to overlap of models with high and low extinction in certain parts of color space.
The formation of early-type dwarf (dE) galaxies, the most numerous objects in clusters, is believed to be closely connected to the physical processes that drive galaxy cluster evolution, like galaxy harassment and ram-pressure stripping. However, the
actual significance of each mechanism for building the observed cluster dE population is yet unknown. Several distinct dE subclasses were identified, which show significant differences in their shape, stellar content, and distribution within the cluster. Does this diversity imply that dEs originate from various formation channels? Does cosmological formation play a role as well? I try to touch on these questions in this brief overview of dEs in galaxy clusters.
Based on the wealth of multiwavelength imaging data from the SDSS, we investigate whether dwarf and giant early-type galaxies in the Virgo cluster follow a continuum in their structural parameters and their stellar population characteristics. More sp
ecifically we study the relation between size and brightness for the galaxies and their color magnitude relation. In both cases, we find noticeable deviations from a simple joint behavior of dwarfs and giants. We discuss these findings in the light of the different formation mechanisms commonly assumed for dwarf and giant early types, thereby taking into account the existence of several distinct early-type dwarf subclasses. By comparing our results to a semianalytic model of galaxy formation, we argue that the analyzed relations might be reproduced by processes that form dwarfs and giants altogether. The work presented here is based on Janz & Lisker 2008, 2009.
In this letter we present a study of the color magnitude relation of 468 early-type galaxies in the Virgo Cluster with Sloan Digital Sky Survey imaging data. The analysis of our homogeneous, model-independent data set reveals that, in all colors (u-g
, g-r, g-i, i-z) similarly, giant and dwarf early-type galaxies follow a continuous color magnitude relation (CMR) that is best described by an S-shape. The magnitude range and quality of our data allows us to clearly confirm that the CMR in Virgo is not linear. Additionally, we analyze the scatter about the CMR and find that it increases in the intermediate-luminosity regime. Nevertheless, despite this observational distinction, we conclude from the similarly shaped CMR of semi-analytic model predictions that dwarfs and giants could be of the same origin.
Various early-type dwarf galaxies with disk features were identified in the Virgo cluster, including objects that display weak grand-design spiral arms despite being devoid of gas. Are these still related to the classical dEs, or are they a continuat
ion of ordinary spiral galaxies? Kinematical information of acceptable quality is available for one of these galaxies, IC 3328. We perform an investigation of its dynamical configuration, taking into account the effect of asymmetric drift, and using the Toomre parameter as well as density wave considerations. The derived mass-to-light ratios and rotational velocities indicate the presence of a significant dynamically hot component in addition to the disk. However, unambiguous conclusions need to await the availability of further data for this and other early-type dwarfs with spiral structure.
The evolution of number density, size and intrinsic colour is determined for a volume-limited sample of visually classified early-type galaxies selected from the HST/ACS images of the GOODS North and South fields (version 2). The sample comprises 457
galaxies over 320 arcmin2 with stellar masses above 3E10 Msun in the redshift range 0.4<z<1.2. Our data allow a simultaneous study of number density, intrinsic colour distribution and size. We find that the most massive systems (>3E11 Msun) do not show any appreciable change in comoving number density or size in our data. Furthermore, when including the results from 2dFGRS, we find that the number density of massive early-type galaxies is consistent with no evolution between z=1.2 and 0, i.e. over an epoch spanning more than half of the current age of the Universe. Massive galaxies show very homogeneous **intrinsic** colour distributions, featuring red cores with small scatter. The distribution of half-light radii -- when compared to z=0 and z>1 samples -- is compatible with the predictions of semi-analytic models relating size evolution to the amount of dissipation during major mergers. However, in a more speculative fashion, the observations can also be interpreted as weak or even no evolution in comoving number density **and size** between 0.4<z<1.2, thus pushing major mergers of the most massive galaxies towards lower redshifts.
(Abridged) The formation of massive spheroidal galaxies is studied on a visually classified sample of 910 galaxies extracted from the ACS/HST images of the GOODS North and South fields (0.4<z<.5). Three key observables are considered: comoving number
density, internal colour distribution; and the Kormendy relation. The comoving number density of the most massive galaxies is found not to change significantly with redshift. One quarter of the whole sample of early-types are photometrically classified as blue galaxies. On a volume-limited subset out to z<0.7, the average stellar mass of the blue ellipticals is 5E9Msun compared to 4E10Msun for red ellipticals. On a volume-limited subsample of bright galaxies (Mv<-21) out to z=1.4 we find only 4% are blue early-types, in contrast with 26% for the full sample. The intrinsic colour distribution correlates overall bluer colours with **blue cores** (positive radial gradients of colour), suggesting an inside-out process of formation. The redshift evolution of the observed colour gradients is incompatible with a significant variaton in stellar age within each galaxy. The slope of the Kormendy relation in the subsample of massive galaxies does not change between z=0 and z=1.4.
In this letter we present a study of the size luminosity relation of 475 early-type galaxies in the Virgo Cluster with Sloan Digital Sky Survey imaging data. The analysis of our homogeneous, model-independent data set reveals that giant and dwarf ear
ly-type galaxies do not form one common sequence in this relation. The dwarfs seem to show weak or no dependence on luminosity, and do not fall on the extension of the rather steep relation of the giants. Under the assumption that the light profile shape varies continuously with magnitude, a curved relation of size and magnitude would be expected. While the galaxies do roughly follow this trend overall, we find that the dwarf galaxies are significantly larger and the low-luminosity giants are significantly smaller than what is predicted. We come to the conclusion that in this scaling relation there is not one common sequence from dwarfs to giants, but a dichotomy which can not be resolved by varying profile shapes. The comparison of our data to a semi-analytic model supports the idea of a physical origin of this dichotomy.
